1. Field of the Invention
The present invention generally relates to nanoparticle synthesis, and more particularly to size controlled synthesis of iron-based nanoparticles, especially iron oxide, iron sulfide nanoparticle materials that can have many important technological applications.
2. Description of the Related Art
Magnetite, Fe3O4, is one of the three common iron oxides, FeO, Fe2O3 and Fe3O4, which have found many important technological applications. Magnetic iron oxide nanoparticle dispersions, commercially known as “Ferrofluid”, have been used widely in, for example, rotary shaft sealing for vacuum vessels, oscillation damping for various electronic instruments, and position sensing for avionics, robotics, machine tool, and automotive [K. Raj, R. Moskowitz, J. Magn. Mag Mater., 85, 233 (1990).]. The magnetite is a semimetallic material. Its dark colored particle dispersion has been used in printing applications as high quality toners or inks [U.S. Pat. No. 4,991,191, U.S. Pat. No. 5,648,170, and U.S. Pat. No. 6,083,476, incorporated herein by reference]. Magnetite dispersion is also useful for the manufacture of liquid crystal devices, including color displays, monochromatic light switches, and tunable wavelength filter [U.S. Pat. No. 3,648,269, U.S. Pat. No. 3,972,595, and U.S. Pat. No. 5,948,321, U.S. Pat. No. 6,086,780, and U.S. Pat. No. 6,103,437, incorporated herein by reference]. As a semiconducting ferrimagnet with high Curie temperature (858 K), the magnetite has shown great potential in tunneling device fabrication. [G. Gong, et al, Phys. Rev. B, 56, 5096(1997). J. M. D. Coey, et al, Appl. Phys. Lett., 72, 734(1998). X. Li, et al, J. Appl. Phys., 83, 7049(1998). T. Kiyomura, et al, J. Appl. Phys., 88, 4768(2000). R. G. C. Moore, et al, Physica E, 9, 253(2001). S. Soeya, et al, Appl. Phys. Lett., 80, 823(2002).] The use of magnetite nanoparticles in clinical medicine is an important field in diagnostic medicine and drug delivery. Magnetite nanoparticles, with size of 10–20 nm, are superparamagnetic. These particles interfere with an external homogeneous magnetic field and can be positioned magnetically in a living body, facilitating magnetic resonance imaging (MRI) for medical diagnosis [U.S. Pat. No. 6,123,920, U.S. Pat. No. 6,048,515, U.S. Pat. No. 6,203,777, U.S. Pat. No. 6,207,134, incorporated herein by reference, D. K. Kim, et al, J. Magn. Mag. Mater., 225, 256(2001)], and AC magnetic field induced excitation for cancer therapy [U.S. Pat. No. 6,165,440, U.S. Pat. No. 6,167,313, incorporated herein by reference, A. Jordan, et al, J. Magn. Mag. Mater., 201, 413(1999)].
All of these medicinal and technological applications of magnetic iron oxide fluids require that the magnetic particle size is within the single domain size range and the overall particle size distribution is narrow so that the particles have uniform physical properties, biodistribution, bioelimination and contrast effects. For example, for medicinal applications, mean particle sizes should generally be in the range 2 to 15 nm and, for use as blood pool agents, the mean overall particle size including any coating materials should preferably be below 30 nm. However, producing particles with the desired size, acceptable size distribution without particle aggregation has constantly been a problem.
Two general methods for producing magnetite ferrofluid have been used in the prior art. In the first method, the magnetic fluid was prepared by grinding of magnetite in a ball mill for a long time with a surfactant and carrier solvent, as exemplified in U.S. Pat. No. 3,215,572, and U.S. Pat. No. 3,917,538, incorporated herein by reference. In the second approach, stable dispersion of magnetite fluid was obtained by transferring co-precipitated magnetite covered with an oleate monolayer into a non-polar solvent. The main characteristics of this method are to obtain an ultrafine magnetic oxide by a chemical reaction from the aqueous solution containing ferrous (Fe2+) and ferric (Fe3+) ions, and to accomplish strong adsorption of surfactants on the magnetic particles in a water solution, as exemplified in U.S. Pat. No. 4,019,994, U.S. Pat. No. 4,855,079, U.S. Pat. No. 6,086,780, incorporated by reference, and other publications [Y. S. Kang, et al., Chem. Mater. 8, 2209(1996). C.-Y. Hong, et al, J. Appl. Phys. 81, 4275(1997). T. Fried, et al, Adv. Mater. 13, 1158(2001).]. This method does not need a long preparation time like the grinding method and is suitable for mass production of magnetic fluid. But, it does need constant adjustments on pH value of the solution to ensure the particle formation and stabilization. Recently, a third sonochemical synthesis of Fe3O4 from Fe(II) salt was reported [R. Vijayakumar, et al, Materials Sci. Eng. A286, 101(2000). G. B. Biddlecombe, et al., J. Mater. Chem., 11,2 937(2001).]. The major disadvantage of all these techniques is heterogeneity in the size distribution of the resulting magnetic particles, the composition of these particles, and/or the interaction forces between the particles. The process towards smaller magnetite nanocrystals has very limited success.